issue warning if internal library name is used across packages

[avsm]

A common failure mode when publishing to opam is that a project depends on an internal library name rather than the public one. This in turn causes a build failure when built with -p.

For example:
https://github.com/ocurrent/ocluster/pull/54/files

There doesn't seem to be a good reason to use private library names across different packages. This might be a good candidate to deprecate and warn about.

[rgrinberg]

Actually, this is just a bug. We can just easily map such names. I solved this problem for package/private libraries. I'll just extract the fix into its own PR.

[rgrinberg]

Fixed by #3729